1. Field of the Invention
This invention relates generally to analytical instruments and more particularly to instruments that detect trace amounts of gases.
2. Background of the Technical Art
Plasma deposition of silicon nitride, silicon dioxide or amorphous silicon is a common processing step in fabrication of integrated circuits (ICs) in active matrix liquid crystal displays. Plasma deposition of such material usually occurs at low pressures, promoted by the presence of energy from a radiofrequency discharge or similar energy source.
Plasma deposition differs from deposition of pyrolytic silicon nitride and similar insulator materials by chemical vapor deposition (CVD) techniques in several respects. First, considerable bond scrambling occurs, wherein some bonds are broken and other bonds, which give rise to different molecular species, are formed. Second, plasma-deposited silicon nitride has no fixed stoichiometry (such as Si.sub.3 N.sub.4) and is more accurately described as a polysilazane, Si.sub.x N.sub.y H.sub.z, containing varying combinations of Si, N and H.
Semiconductor device performance often degrades after silicon nitride is used as a final passivation film for an IC, or after an anneal cycle to reduce plasma damage or improve electrical contact resistance. Silicon nitride has low hydrogen permeability so that free hydrogen can become trapped under a silicon nitride layer, or within this layer. Hydrogen evolution during subsequent high temperature processing is often the cause of such degradation, but IC manufacturers have had only limited methods to determine how such evolution occurs. The presence of hydrogen in plasma-deposited silicon nitride is confirmed by absorption spectrum analysis of such a compound, which manifests N--H and Si--H stretching bonds at the well known wavenumbers of 3350 cm.sup.-1 and 2160 cm.sup.-1, respectively.
The hydrogen evolved during subsequent application of high temperatures may be lightly bound interstitial hydrogen or may arise from bond breaking in compounds such as SiH and SiH.sub.2 in hydrogenated amorphous silicon (a-Si:H), NH and NH.sub.2 in nitride, and SiOH and trapped H.sub.2 O in oxide films. The particular bonds that are present are important because different bonds are broken in different temperature ranges, and a temperature cycle for a particular process may include some or all of these temperature ranges.
Plasma-deposited silicon nitride, used as an encapsulant, as a gate dielectric and as an etch stop for IC fabrication, has been observed to degrade after cycling through the high temperatures used for annealing, for cure of plasma damage, for improvement of contact resistance, or for general semiconductor processing. The presence of hydrogen has been determined to be responsible, in large part, for this degradation. Plasma-deposited silicon nitride has no fixed stoichiometry and is best described by the general chemical formula Si.sub.x H.sub.y N.sub.z, with x, y and z variable. Plasma-deposited silicon nitride often contains 10-30 atomic percent hydrogen, as interstitial hydrogen or in a chemical form such as SiH.sub.2, NH.sub.2, SiH or NH. The bonding strengths of the four molecules SiH.sub.2, NH.sub.2, SiH or NH differ and are known to increase in that order. Silicon nitride, once formed, has low permeability to hydrogen and H.sub.2 O and can trap hydrogen evolved from the layer below the passivation film. It is difficult to determine the source(s) of the hydrogen that degrades the silicon nitride film, and no equipment is presently available that allows identification of the source(s) of this hydrogen.
A similar problem is encountered in use of amorphous silicon films for active matrix LCDs, solar cells and other photo-imaging tools. Amorphous silicon is often prepared using plasma-enhanced chemical vapor deposition, and the hydrogen content in a-Si:H is typically 10 atomic percent. The stoichiometry of a-Si:H is also not fixed, and this compound is best described by the chemical formula SiH.sub.y with y variable. Free hydrogen in a-Si:H passivates the dangling bonds and reduces the spin density so that the field effect charge carrier mobility in a thin film transistor increases. The carrier mobility value determines whether the a-Si:H film is suitable for AMCLD applications. Thus, it is important to determine the hydrogen content and hydrogen source(s) for amorphous silicon films.
Silicon nitride is used as a gate dielectric etch stopper and for passivation in AMCLD. Device reliability heavily depends upon the hydrogen bond stability in the silicon nitride film.
Lanford and Rand, in Jour. Appl. Phys., vol. 49 (1978) pp. 2473-2477, have determined the hydrogen content of plasma-deposited silicon nitride, using a nuclear reaction H+N.sup.15 --C.sup.12 +He.sup.4 +.gamma.-ray (4.43 MeV) in which the number of 4.43 MeV .gamma.-rays emitted is proportional to the concentration of H present. Over a ratio range Si/N=0.7-1.4, the hydrogen content varied between 1.6.times.10.sup.21 and 2.1.times.10.sup.21 (cm.sup.-3). This method of determining the amount of hydrogen present is expensive and does not, by itself, indicate the chemical forms in which the hydrogen is bound in the plasma-deposited material. As a result, this technique will not determine whether or not the hydrogen present will cause problems.
Beyer et al, in Jour. Appl. Phys., vol. 53 (1982) pp. 8745-8754, and in Materials Research Society Symposium, vol. 219 (1991) pp. 81-86, discuss hydrogen stability in hydrogenated amorphous semiconductor compounds, such as a-Si:H, a-Ge:H, a-Si--Ge:H, a-Si--N:H and a-Ge--N:H. The authors heat the individual hydrogenated compounds in a vacuum and use a quadrupole mass analyzer to identify hydrogen evolution and evolution rate at temperatures T=200.degree.-800.degree. C. Time rate of change of gas pressure is determined for particular species. One or more peaks of hydrogen evolution rate occur for amorphous silicon and nitride alloys. The technique is applied to study of diffusion characteristics of hydrogen for a-Si:H and silicon nitride but is not applied to study total hydrogen concentration and bonding.
Another technique sometimes applied, Fourier transfer infrared spectroscopy (FTIR), provides measurements on bonded hydrogen only. The Si--H and N--H absorption bands are well isolated, but calibration is critical and must usually be redone each time the spectroscopic instrument is used. The FTIR approach gives no information about the presence of interstitial hydrogen. The quantitative results are not always clear here, and FTIR provides no direct information on hydrogen evolution.
Sakka et al, in Appl. Phys. Lett., vol. 55 (1989) pp. 1068-1070, discuss hydrogen evolution from plasma-deposited, hydrogenated amorphous silicon films prepared from an SiH.sub.4 /H.sub.2 mixture, using apparatus similar to that of Beyer et al, supra. Sakka et al indicate that hydrogen evolution is determined in part by the deposition conditions for the SiH.sub.4 fraction rather than by diffusion characteristics. The Sakka et al technique is not used to determine hydrogen concentration.
It is desirable to determine the total amount of hydrogen present in a material that will be processed over a temperature range including high temperatures. It is also desirable to determine the hydrogen bonding strength and the chemical forms in which the hydrogen is bound on the IC. It is further desirable to estimate the temperature range or ranges at which large amounts of this hydrogen will be released during subsequent processing of the IC.